In a spread spectrum communication system, downlink transmissions from a base station to a mobile station include a pilot channel and a plurality of traffic channels. The pilot channel is decoded by all users. Each traffic channel is intended for decoding by a single user. Therefore, each traffic channel is encoded using a code known by both the base station and the mobile station. The pilot channel is encoded using a code known by the base station and all mobile stations. Spread spectrum encoding of the pilot and traffic channels spreads the bandwidth of transmissions in the system.
One example of a spread spectrum communication system is a cellular radiotelephone system according to Telecommunications Industry Association/Electronic Industry Association (TIA/EIA) Interim Standard IS-95, “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (IS-95). Individual users in the system use the same frequency but are distinguishable from each other through the use of individual spreading codes. IS-95 is an example of a direct sequence code division multiple access (DS-CDMA) communication system. In a DS-CDMA system, transmissions are spread by a pseudorandom noise (PN) code. Data is spread by chips, where the chip is the spread spectrum minimal-duration keying element.
Other spread spectrum systems include radiotelephone and data systems operating at various frequencies and utilizing various spreading techniques. Among these additional systems are third-generation spread spectrum communication systems (3G) and wideband code division multiple access systems (W-CDMA).
Mobile stations for use in spread spectrum communications systems have employed RAKE receivers. A RAKE receiver is a form of a matched filter receiver that includes one or more receiver fingers independently receiving radio frequency (RF) signals. Each finger despreads the traffic channel to form estimates of the traffic symbols. Each finger also despreads and filters the pilot channel to form estimates of the channel gain and phase corresponding to the finger. The traffic symbol estimates of the receiver fingers are combined to produce a received signal. A RAKE receiver combines multipath rays using the complex conjugate of the channel estimates and thereby exploits channel diversity. Generally, the RAKE receiver fingers are assigned to the strongest set of multipath rays.
A limitation on the performance of a RAKE receiver is multiple-access interference or noise at the receiver. Generally, there are two sources of multiple-access interference on the forward link, i.e., from the base station to the mobile station. The first source of interference, typically referred to as intra-cell interference, is the signal originating from the same sector of the same base station as the signal of interest, and results from multipath in the channel between the transmitter and the mobile. The multipath destroys the orthogonality of the transmitted signal, so that signals transmitted to other mobiles interfere with the signal of interest. The second source of multiple-access interference is interference from other sectors, both those sectors in soft-handoff with the mobile station and those not in soft-handoff with the mobile station. The signals transmitted from neighboring sectors are not orthogonal with the signal of interest, regardless of channel, so some multiple-access interference is always introduced at the receiver. Under these conditions, the RAKE receiver performance is limited by multiple access interference.
An alternative structure that may be used as a receiver in a DS-CDMA system is a minimum mean square error (MMSE) equalizer. A receiver based on an MMSE equalizer (MMSE receiver) is disclosed and described in commonly-assigned U.S. Pat. No. 6,175,588, the disclosure of which is hereby expressly incorporated herein by reference. A MMSE receiver can effectively suppress both intra-cell (Ior) and other-cell (Ioc) multiple-access interference on a DS-CDMA downlink. As a result, depending on the specific channels and on the ratio of Ioc/Ior, the performance of the MMSE receiver can be in excess of several decibels (dB) better than the RAKE receiver. In DS-CDMA systems gains are important because the link capacity scales with the inverse of the signal-to-noise ratio required to support the link.
A problem of the MMSE receiver is that adaptive implementations (least mean square “LMS”, recursive least square “RLS”, or multi-stage Wiener) have difficulty tracking high-speed Doppler. Furthermore, if the equalizer does not adequately track the channel, the performance of the equalizer can be worse than the performance of the RAKE receiver, which is very robust.
Thus, there is a need for a spread spectrum receiver that adapts the advantages of a RAKE receiver and an MMSE receiver in an efficient architecture.